The present invention relates to an impregnated cathode for use in an electron tube such as Braun tube or a camera tube, and more particularly to an impregnated cathode having a porous metal body of improved construction.
An impregnated cathode has a porous metal body which is impregnated with an electron emissive material composed majorly of an oxide of rare earth metal such as Ba. As this porous metal body, there has usually been used a porous body of refractory metal such as tungsten, molybdenum, tantalum, rhenium or nickel. The porous metal body impregnated with such electron emissive material has a sleeve mounted thereon and a heater attached thereto, thus fabricating the desired impregnated cathode. The sleeve is usually made of refractory metal such as molybdenum, tantalum or tungsten. On the other hand, the heater is usually made of a tungsten wire and is formed thereon with an alumina coating layer all over the tungsten wire.
Now, when the impregnated cathode having the construction thus far described is operated until its temperature reaches a normal operating temperature, i.e., 1000.degree. C. the Ba compound is liberated from the porous metal body to the heater so that it steals into the insulating alumina coating layer, which is formed on the surface of the heater wire, to deteriorate the insulating property of the heater until the impregnated cathode becomes unfit for use. In order to obviate such disadvantage, a partition plate of refractory metal has been disposed in contact with the heater side of the porous metal body in accordance with the prior art, thus preventing Ba or its compound from being liberated. FIG. 1 is a sectional view showing one example of the impregnated cathode according to the prior art. Indicated at reference numeral 1 is a porous tungsten body which is impregnated with an electron emissive material containing a Ba oxide or the like. Indicated at numeral 2 is a sleeve which is made of molybdenum or tantalum. Indicated at numeral 3 is a partition which is also made of molybdenum or tantalum and which is formed into such a cup shape as is suitable to be adhered to the sleeve 2 and the porous tungsten body 1. The partition 3 is welded or brazed at its adhered position 4 to the sleeve 2 and is brazed at its adhered position 5 to the porous tungsten body 1. Incidentally, illustration of the heater is omitted from FIG. 1.
The conventional impregnated cathode having the metal partition described in the above can be fabricated with relative ease either in case a porous metal body having such a relatively large size as makes the diameter of the cathode as large as several millimeters is used or in case the sleeve is made so thick as has a thickness as large as about 0.1 mm. However, both in the case of a small cathode such as a cathode having a diameter at most 1.5 mm for use in the Braun tube or the camera tube and in the case of a thin cathode made as thick as about 20 .mu.m with a view to reducing the power consumption, the impregnated cathode having the construction with the aforementioned metal partition is difficult to produce with the resultant increase in its production cost. In the impregnated cathode according to the aforementioned prior art, moreover, it is difficult to braze the porous metal body and the partition in a complete manner all over their contacting surfaces, and there is established a gap inbetween thereby to reduce the thermal efficiency so that the emission current density is reduced. Still moreover, since the respective cathodes are different among their gap or bonded areas, there arises a large dispersion in the thermal efficiency and accordingly in the emission current density.
Generally speaking, the impregnated cathode has its Ba or the oxide thereof evaporated during its operation from the impregnating electron emissive material so that the content in the impregnating electron emissive material is reduced with the time lapse until the impregnated cathode becomes unfit for use. In order to elongate the lifetime of the impregnated cathode, therefore, it is sufficient that the quantity of impregnation of the electron emissive material is increased. For this purpose, it is necessary either to enlarge the porous metal body or to increase the porosity of the same. However, in the former case of the enlarged porous metal body, the power consumption by the heater is increased whereas in the latter case of the porous metal body having the increased porosity the rate of evaporation of the electron emissive material is increased, both of the cases being undesired.